A term of encouragement often used to drunkenly encourage equally intoxicated acquaintances. Rarely used on the subject of encouragement's actual birthdate. Commonly preceeded by "Go (name)" and occasionally followed by, "no it's not, go anyway" when spoken by those in the 8-13 year old age bracket (who are hopefully not drunk).

Used by rappers and dorky white kids, its your birthday means that you have completed something and your friends (if you have any) and proud of you, resulting in them saying "its your birthday" meaning "Congrats! You've done some ordinary thing that many other people do, but I'm going to use the phrase "its your birthday" to sound cool, and hip, because I saw it on MTV!!!!"
Yeah.